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United States Patent |
5,108,518
|
Fukui
,   et al.
|
April 28, 1992
|
Method of producing thin high carbon steel sheet which exhibits
resistance to hydrogen embrittlement after heat treatment
Abstract
A method of manufacturing a thin steel sheet of high carbon steel which has
a high level of strength and exhibits excellent resistance to hydrogen
embrittlement after heat treatment is disclosed. The steel consists
essentially of, by weight %:
C: 0.30-0.70%,
Si: 0.10-0.70%,
Mn: 0.05-1.00%,
P: not greater than 0.030%,
S: not greater than 0.020%,
Cr: 0.50-2.00%,
Mo: 0.10-0.50%,
Ti: 0.005-0.10%,
Nb: 0.005-0.100%,
sol. Al: not greater than 0.10%,
N: greater than 0.0020%, but not greater than 0.015%,
optionally B: 0.0005-0.0020%, and
a balance of iron and incidental impurities, the method comprising the
steps of:
hot rolling a steel having the above defined chemical composition with a
finishing temperature of 800.degree. C. or higher;
immediately after finishing the hot rolling cooling the hot rolled steel at
a cooling rate of 5.degree.-40.degree. C./second to a temperature range of
500.degree.-700.degree. C.; and
coiling the hot rolled steel at a temperature of 450.degree.-650.degree. C.
Inventors:
|
Fukui; Kiyoshi (Nishinomiya, JP);
Okamoto; Atsuki (Ashiya, JP)
|
Assignee:
|
Sumitomo Metal Industries, Ltd. (Osaka, JP)
|
Appl. No.:
|
626830 |
Filed:
|
December 13, 1990 |
Foreign Application Priority Data
Current U.S. Class: |
148/603 |
Intern'l Class: |
C21D 008/00 |
Field of Search: |
148/12 F,12 R,330,334
|
References Cited
U.S. Patent Documents
4406711 | Sep., 1983 | Nagumo et al. | 148/12.
|
4472208 | Sep., 1984 | Kunishige | 148/12.
|
Foreign Patent Documents |
0023025 | Feb., 1982 | JP.
| |
0211526 | Nov., 1984 | JP.
| |
1159534 | Jul., 1986 | JP.
| |
0145718 | Jun., 1988 | JP.
| |
Primary Examiner: Dean; R.
Assistant Examiner: Ip; Sikyin
Attorney, Agent or Firm: Burns, Doane, Swecker & Mathis
Claims
What we claim is:
1. A method of manufacturing a thin steel sheet of high carbon steel which
has a high level of strength and exhibits excellent resistance to hydrogen
embrittlement after heat treatment and which has the following chemical
composition which consists essentially of, by weight %:
C: 0.30-0.70%,
Si: 0.10-0.70%.
Mn: 0.05-1.00%,
P: not greater than 0.030%,
S: not greater than 0.020%,
Cr: 0.50-2.00%,
Mo: 0.10-0.50%,
Ti: 0.005-0.10%,
Nb: 0.005-0.100%,
B: 0-0.0020%,
sol. Al: not greater than 0.10%,
N: greater than 0.0020%, but not greater than 0.015%,
and a balance of iron and incidental impurities, the method comprising the
steps of:
hot rolling a steel having said chemical composition with a finishing
temperature of 800.degree. C. or higher;
immediately after finishing the hot rolling cooling the hot rolled steel
sheet at a cooling rate of 5.degree.-40.degree. C./second to a temperature
range of 500.degree.-700.degree. C.,
coiling the hot rolled steel sheet at a temperature of
450.degree.-650.degree. C.; and
carrying out at least one cold rolling step with a reduction of 20-80% and
box annealing at a temperature range of (Ac.sub.1 -50) to (Ac.sub.1
+30).degree. C.
2. A method of manufacturing a thin steel sheet as set forth in claim 1,
wherein the B content is 0.0005-0.0020%.
3. A method of manufacturing a thin steel sheet as set forth in claim 1,
wherein the Mn content is restricted to not greater than 0.80%.
4. A method of manufacturing a thin steel sheet as set forth in claim 1,
wherein the S content is restricted to not greater than 0.010%.
5. A method of manufacturing a thin steel sheet as set forth in claim 1,
wherein the ratio of Ti/Nb is 0.3-0.7.
6. A method of manufacturing a thin steel sheet as set forth in claim 1,
wherein the cooling rate is restricted to 10.degree.-20.degree. C./second.
7. A method of manufacturing a thin steel sheet as set forth in claim 1,
further comprising a step of austempering the steel at a temperature and
time sufficient to provide a bainitic grain structure.
8. A method of manufacturing a thin steel sheet as set forth in claim 7,
wherein the steel has a tensile strength of at least 120 kgf/mm.sup.2
after the austempering step.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a method of manufacturing a thin steel
sheet of high carbon steel, and particularly to a method of manufacturing
a carbon steel sheet which has a very fine crystal grain structure after
heat treatment and which is superior to a conventional thin steel sheet
with respect to resistance to mechanical shock and wear, and which can
successfully prevent cracking caused by hydrogen infiltrated into the
steel during use.
The steel sheet of the present invention is easy to produce and easy to
work, and therefore, it is suitable for forming a variety of articles
including chain elements, gear members, clutch members, buckles for seat
belts, and washers.
In general, such articles are manufactured from a high carbon steel such as
S30CM, S70CM, SK7M, or SK4M and a low alloy high carbon steel such as
SCM435 or SCM445 as specified in JIS G 3311. These high carbon steels and
low alloy, high carbon steels are, if necessary after hot rolling and
descaling by pickling, subjected to cold rolling with a certain degree of
reduction in area as well as spheroidizing by heating the steel at a
temperature of around the Ac.sub.1 point for an extended period of time in
order to improve the accuracy of the thickness of the sheet and
formability including its suitability for blanking, bending, and pressing
which are usually carried out by customers. After forming into a final
shape, heat treatment such as quenching and tempering, austempering, and
the like is performed to harden the article being manufactured, resulting
in improvement in the resistance to wear and mechanical shock.
Therefore, steel sheet for use in manufacturing these articles must exhibit
a relatively high degree of strength as well as shock- and wear-resistant
properties only after heat treatment. For this purpose it is necessary to
employ steel with a high carbon content. Since the shock- and
wear-resistant properties of the final articles are influenced by heat
treatment conditions, and especially the tempering temperature, steel
sheet "as quenched" or steel sheet "tempered at 650.degree. C. or lower"
(usually 180.degree.-450.degree. C.) in the case of quenched material, or
steel sheet austempered at a temperature of 500.degree. C. or lower
(usually 200.degree.-450.degree. C.) in the case of austempered material
are carefully chosen depending on the properties required for the use.
However, since thin steel sheet of high carbon steel, especially that with
a higher content of carbon suffers from an increase in strains in the
steel and precipitation of a large amount of carbides in the course of
heat treatment, it is inevitable that the shock-resistance as well as the
resistance to hydrogen embrittlement are insufficient, even if the heat
treatment conditions are determined carefully.
SUMMARY OF THE INVENTION
However, according to the experience of the inventors, when such a high
strength steel is used, cracking occurs in an area where stress is
concentrated during use thereof, and observation of the fractured surface
shows that the fracture is caused by intergranular fracture. The inventors
of the present invention found that such cracking is caused by the
hydrogen which penetrates into the steel during its use. This is hereunder
called hydrogen embrittlement.
Therefore, it is a general object of the present invention to provide a
method of manufacturing a thin steel sheet of high carbon which exhibits
excellent resistance to mechanical shock and wear as well as to hydrogen
embrittlement.
In order to avoid hydrogen embrittlement it is necessary to suppress an
increase in strains due to a high content of carbon. For this purpose, it
is advisable to employ a CrMo system, low-alloy steel, such as SCM435, and
SCM445, which contains a reduced amount of carbon.
Furthermore, it is also advisable to prepare a fine austenitic crystal
grain structure and to prevent propagation of cracking within the steel.
For this purpose it is necessary to suitably adjust the chemical
composition of the steel including chemical components such as Al and N.
In particular, in order to refine the austenitic grain structure it is
common to precipitate fine particles of AlN and the like during slab
heating, or soaking before quenching or austempering.
However, in order to provide more finely divided austenitic crystal grains,
it is necessary to increase the amount of precipitates other than AlN and
the like. The inventors conceived of adding Ti and Nb to precipitate TiN,
TiC, Ti(CN), NbC, Nb(CN), or TiNb(CN) the presence of which can promote
refinement of the crystal grains efficiently.
There is a current trend to improve shock-resistance and the resistance to
hydrogen embrittlement by austempering instead of by quenching and
tempering. It is also required to further shorten the length of time
required for achieving heat treatment including quenching and tempering,
and austempering for automobile components because an increasingly large
number of components are used for assembling a modern automobile and the
processing time for each component should be minimized. However, in the
case of the above-described low alloy steel, when the heating time for
preparing an austenitic structure before austempering is shortened,
sometimes transformation from a pre-structure, i.e., ferrite-pearlite
structure into an austenitic structure does not take place sufficiently,
resulting in a local concentration of carbon, which causes formation of a
mixed structure of martensite and bainite after austempering. The presence
of such a mixed structure adversely affects the shock-resistance and the
resistance to hydrogen embrittlement. Thus, it is necessary to prevent
formation of such a mixed structure but to form a uniform bainite
structure so as to improve such properties.
It is a specific object of the present invention to provide an economical
and practical method of manufacturing a thin steel sheet of high carbon
steel which has finely divided crystal grains and exhibits excellent
resistance to mechanical shock and wear, as well as to hydrogen
embrittlement.
The inventors of the present invention made the following discoveries.
(a) In the past, it was thought to be impossible to completely prevent a
high strength steel from suffering from hydrogen embrittlement and fatigue
fracture. However, even for such a high strength steel, when a specific
amount of Nb (0.005-0.100%) is incorporated into the steel, austenitic
grains are finely divided and the cracking due to hydrogen embrittlement
is suppressed markedly.
(b) Additionally, 0.005-0.10% of Ti is added, Ti(CN) and TiNb(CN) are
formed during slab heating, or soaking before quenching or austempering to
markedly suppress the grain growth of austenite phase.
(c) When the content of P is reduced to 0.030% or less, the amount of P
precipitated along austenitic grain boundaries is also reduced, resulting
in an improvement in the toughness of the resulting material. The presence
of a large amount of P precipitated along grain boundaries causes grain
boundary embrittlement, sometimes resulting in brittle fracture.
(d) It is known that when a suitable amount of B is added, B precipitates
along grain boundaries preferentially to P so that the precipitation of P
is suppressed. Therefore, it has also been proposed to incorporate B in
steel so as to prevent the precipitation of P along grain boundaries. The
present inventors found that the addition of B is also effective to avoid
hydrogen embrittlement by further strengthening grain boundaries.
(e) When the S content is reduced to 0.020% or less, a reduction in the Mn
content can greatly improve toughness because formation of MnS is
eliminated. In contrast, when the Mn content is high, due to the presence
of a high amount of Mn, the precipitation of P along grain boundaries is
promoted. However, when the Mn content is lowered, the precipitation of P
along grain boundaries can be successfully prevented. Usually when the Mn
content is small, it is expected that hardenability is degraded, but the
steel sheet manufactured by the present invention is very thin, and
hardenability is not so serious a problem. In addition, due to
incorporation of Cr and Mo, the strength of the resulting steel is
ensured.
(f) Generally, it is inevitable that formability or punchability before
quenching and tempering are degraded for a high carbon steel sheet with
improved toughness. However, when a specific amount of Mo is incorporated
as an alloying element, a degradation in toughness after quenching and
tempering, particularly that called "tempering embrittlement at low
temperatures" can be avoided successfully.
(g) When the finishing temperature is defined as 800.degree. C. or higher,
a ferrite-pearlite grain structure after hot rolling can be further
refined, resulting in a uniform structure after heat treatment with
improvement in the resistance to mechanical shock and hydrogen
embrittlement.
(h) When the cooling rate after hot rolling is defined as 10-40.degree.
C./second, proeutectoid ferrite grains in the hypoeutectoid composition
range can be successfully refined. Therefore, the length of heating can be
shortened in the austenitic temperature range during soaking carried out
before quenching-tempering or austempering.
(i) When a hot rolled sheet is coiled in the temperature range of
550.degree.-650.degree. C., a refinement of the above-mentioned
proeutectoid ferrite grains can be promoted more successfully, resulting
in a shortening of the time for soaking in the austenitic temperature
range before quenching-tempering or austempering.
On the basis of these discoveries, the inventors disclosed steels
containing Nb, Cu, Ti, and B in Japanese Patent Applications No. 311136
and 301815/1988. These steels have excellent toughness, but they are
expensive since they contain Cu which is indispensable for improving the
resistance to hydrogen embrittlement on the surface of a steel sheet.
A steel composition similar to the above-defined one is disclosed in
Japanese Patent Publication No. 35066/1989, in which the N content is
restricted to 0.0020% or less and the P content is restricted to 0.010% or
less so as to obtain clean grain boundaries. In this case, however, the
grain size after quenching-tempering or austempering becomes coarse,
resulting in a degradation in the resistance to shock and hydrogen
embrittlement. In addition, the steel disclosed in that patent application
is mainly used for manufacturing rods, bars, and the like, so that
application does not suggest anything about heat treatment of a thin steel
sheet or about defects associated therewith.
The present inventors have confirmed on the basis of the above-described
discoveries not only that manufacturing costs can be markedly reduced but
also that the resistance to mechanical shock and hydrogen embrittlement
can be improved markedly by (i) eliminating the incorporation of copper,
(ii) restricting the N content to larger than 0.0020% but not larger than
0.015%, (iii) adjusting the steel composition so as to contain a
sufficient amount of Ti, Al, and Nb-containing carbonitride, and further
(iv) adjusting the hot rolling conditions such that a fine
ferrite-pearlite grain structure will be formed.
The present invention resides in a method of manufacturing a thin steel
sheet of high carbon steel which has a high level of strength and exhibits
excellent resistance to hydrogen embrittlement after heat treatment and
which has the following chemical composition which consists essentially
of, by weight%:
C: 0.30-0.70%,
Si: 0.10-0.70%.
Mn: 0.05-1.00%,
P: not greater than 0.030%,
S: not greater than 0.020%,
Cr: 0.50-2.00%,
Mo: 0.10-0.50%,
Ti: 0.005-0.10%,
Nb: 0.005-0.100%,
sol. Al: not greater than 0.10%,
N: greater than 0.0020%, but not greater than 0.015%, optionally B:
0.0005-0.0020%, and
a balance of iron and incidental impurities, the method comprising the
steps of:
hot rolling a steel having the-above defined chemical composition with a
finishing temperature of 800.degree. C. or higher;
immediately after finishing the hot rolling cooling the hot rolled steel
sheet at a cooling rate of 5.degree.-40.degree. C., preferably
10.degree.-40.degree. C./second to a temperature range of
500.degree.-700.degree. C., preferably 550.degree.-650.degree. C.; and
coiling the hot rolled steel sheet at a temperature of
450.degree.-650.degree. C., preferably 550.degree.-650.degree. C.
Optionally, the present invention may further comprise the step of carrying
out one or more times cold rolling with a reduction of 20-80% and box
annealing at a temperature of (Ac.sub.1 -50) to (Ac.sub.1 +30).degree. C.
Thus, according to the present invention, a bainite structure can be
produced in an extremely short time by applying heat treatment such as
austempering.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 through FIG. 4 are graphs showing results of working examples of the
present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The reasons for defining the steel composition which is processed by the
present invention as above will now be described.
(a) C (Carbon)
In order to provide a steel with a satisfactory level of hardness,
strength, hardenability and wear resistance 0.30% or more of carbon is
added. When the carbon content is over 0.70%, formability before heat
treatment is degraded and toughness after heat treatment is much impaired.
Therefore, in the present invention the carbon content is defined as
0.30-0.70%, and preferably 0.40-0.60%.
(b) Si (Silicon)
The addition of Si is not absolutely necessary. When the content of Si is
over 0.70%, there is a tendency for the steel to become hard and brittle
to some extent. The Si content is restricted to not greater than 0.70%. At
least 0.10% of Si is added so as to ensure hardenability.
(c) Mn (Manganese)
The high carbon steel sheet of the present invention which contains Cr and
Mo is mainly used for making gears, chains and the like. In contrast to
conventional wear-resistant steel sheet for general use the content of Mn
is reduced in the present invention so as to improve toughness. Therefore,
it is desirable that the Mn content be so small as to improve toughness.
In fact, when the amount of Mn is over 1.0%, the hardness after heat
treatment is much greater than that required, resulting in a degradation
in toughness. On the other hand, when the Mn content is lower than 0.05%,
the amount of sulfur which is dissolved in the steel increases and
embrittlement during hot working is inevitable, resulting in a degradation
in productability of steel sheet. The Mn content is restricted to
0.05-1.00%, and preferably not greater than 0.80%.
(d) P (Phosphorous)
Phosphorous precipitates along grain boundaries of austenite before
transformation, and it has a great influence on degradation in the
resistance to embrittlement such as intergranular fracture. Therefore, the
lower the phosphorous content the better with respect to toughness.
According to the present invention, the P content is restricted to 0.030%
or less. However, it is preferable to further lower the P content when a
relatively large amount of Si and Mn is incorporated. For this purpose, it
is desirable to restrict the P content to not greater than 0.015%. From
the viewpoint of steelmaking it is desirable to define the lower limit of
the P content as 0.010% to avoid adding to manufacturing costs.
Precipitation of phosphorous along grain boundaries is suppressed by the
addition of boron. This is because boron precipitates along the grain
boundaries in preference to phosphorous, resulting in less precipitation
of phosphorous. Thus, the austenite grain boundaries may be strengthened
by the addition of B. This is the same as when the content of P is
reduced.
(e) S (Sulfur)
The smaller the S content, the more effectively is the precipitation of MnS
suppressed. This is desirable from the viewpoint of improving toughness.
Therefore, the S content is defined as 0.020% or less, and preferably
0.01% or less.
(f) Nb (Niobium)
Nb is effective to refine austenite grains and to improve the toughness of
the steel. Refinement of austenite grains is also very effective at
suppressing hydrogen embrittlement fracture. When Nb is added in an amount
of less than 0.005%, its effects are inadequate. On the other hand, when
Nb is added in an amount of larger than 0.100%, the effectiveness of its
addition saturates. Thus, the Nb content is restricted to 0.005-0.100%.
Preferably, the ratio of Ti/Nb is about 0.3-0.7 so as to form a TiNb
system precipitate.
(g) Cr (Chromium)
Cr is added mainly for the purpose of improving hardenability. When Cr is
added in an amount of larger than 2.0%, the steel becomes hard, resulting
in embrittlement. Therefore, the Cr content is restricted to 0.50-2.00%.
(h) Mo (Molybdenum)
The addition of Mo is important. It is effective for maintaining a high
degree of toughness after heat treatment without degradation in
formability before heat treatment (quenching and tempering) of steel
sheet. Generally, when a steel is tempered at around 300.degree. C. after
quenching, a so-called "low temperature temper brittleness" occurs with a
marked degradation in toughness. However, sometimes it is necessary to
carry out low temperature tempering at around 300.degree. C. in order to
obtain a desirable level of hardness. In fact, there is a tendency for
temper brittleness of a thin steel sheet to be less serious than that of a
thick steel sheet, and that low temperature tempering may be employed for
a thin steel sheet. However, even a very small deterioration in toughness
is inevitable. The addition of Mo in an amount of 0.10% or larger is very
effective for compensating for a reduction in toughness of this level.
When the Mo content is larger than 0.50%, it adds to manufacturing costs,
so the upper limit of Mo content is defined as 0.50%.
(i) Ti (Titanium)
Ti is effective for increasing the hardenability, hardness and tensile
strength of steel by means of forming and finely dispersing TiN or TiC.
Furthermore, Ti combines with Nb to form a complex precipitate TiNb(CN)
which promotes refinement of austenite grains. When B is also added, the
presence of Ti suppresses the precipitation of BN within a grain and the
precipitation of P along grain boundaries to avoid a degradation in the
resistance to mechanical shock and hydrogen embrittlement. However, when
the Ti content is less than 0.005%, the above-mentioned advantages cannot
be obtained. The addition of Ti in an amount of larger than 0.10% adds to
manufacturing costs and results in hardening of the steel. The Ti content
is defined as 0.005-0.10%. It is desirable to add Ti in an amount not
exceeding the amount of Nb in order to form a TiNb system complex
precipitate.
(j) sol. Al
Al is optionally added to steel as a deoxidizing agent. When the content of
Al is over 0.10%, it adds to manufacturing costs and makes the resulting
steel hard. In view of grain size control of an austenitic grain, the
addition of an excess amount of Al is undesirable. Thus, the addition of
Al in an amount of not greater than 0.10% is permissible.
(k) N (Nitrogen)
The incorporation of nitrogen is effective for improving the hardness and
tensile strength of steel. The presence of nitrogen is also effective for
suppressing grain growth of austenitic grains by the formation of AlN,
TiN, and the like, resulting in an improvement in toughness. For this
purpose the N content is restricted to larger than 0.0020%. However, when
the N content is over 0.015%, formability before quenching is impaired
because of an increase in hardness. Therefore, the upper limit is defined
as 0.015%.
(l) B (Boron):
B is an optional element, but it is effective for improving hardenability
and strengthening grain boundaries due to its presence in a dissolved form
along grain boundaries. When B is added in an amount of 0.0005% or more,
the occurrence of brittle fracture is markedly suppressed. However, its
effectiveness saturates when it is added excessively, and manufacturing
costs are unnecessarily increased. Thus, according to the present
invention, the content of B is restricted to not greater than 0.0020%.
(m) Finishing Temperature
The finishing temperature is restricted to 800 .degree. C. or higher to
prevent precipitation of proeutectoid ferrite before finishing rolling.
The upper limit is preferably 880.degree. C. to avoid cracking during
pickling and cold rolling, the cracking being caused by an increase in the
hardness of hot rolled steel sheet.
(n) Cooling Rate for Hot Rolled Steel Sheet:
In order to prepare a fine ferrite-pearlite crystal grain structure, not
only the finishing temperature but also the cooling rate for hot-rolled
steel sheet are important.
In general, it is said that the lower the cooling rate for ferrite the
smaller and coarser the number of grains of proeutectoid ferrite. Such
coarse grains of ferrite adversely affect the refinement of austenite
grain structure and also result in an increase in a heat treatment time
since it takes much time for carbon and alloying elements such as Mn, Cr,
and Mo to diffuse in an austenitic temperature range. In order to avoid
such disadvantages it is necessary to increase the cooling rate for hot
rolled steel sheet. However, when the cooling rate after finishing rolling
is smaller than 5.degree. C./second, refinement of an austenite grain
structure cannot be expected. On the other hand, when the cooling rate is
over 40.degree. C./second, the hardness of the steel sheet increases,
resulting in cracking during pickling and cold rolling.
Thus, the cooling rate after finishing rolling is restricted to
5.degree.-40.degree. C./second. Preferably, the cooling rate is restricted
to 10.degree.-40.degree. C., further preferably to 10.degree.-20.degree.
C./second, since at a cooling rate of higher than 20.degree. C./second the
hot-rolled steel sheet becomes embrittled, sometimes resulting in cracking
during pickling.
(o) Coiling Temperature
After cooling to 500.degree.-700.degree. C., preferably to
550.degree.-650.degree. C., the resulting hot rolled steel sheet is coiled
at a temperature of 450.degree.-650.degree. C., preferably
550.degree.-650.degree. C. When the coiling temperature is over
650.degree. C., proeutectoid ferrite is made coarse even if it is cooled
under the above conditions so that an extended period of time is required
for heat treatment after shaping. On the other hand, when it is coiled at
a temperature lower than 450.degree. C., the steel sheet is hardened,
resulting in cracking during pickling and cold rolling. Therefore, in the
present invention the coiling temperature is restricted to 450-650.degree.
C., preferably 550.degree.-650.degree. C.
(p) Cold rolling
According to a preferred embodiment of the present invention, the
thus-resulting hot rolled steel sheet is, if necessary, subjected to cold
rolling and then box annealing. The reduction in thickness during cold
rolling is restricted to 20% or larger so as to ensure the required
accuracy of the final thickness. The upper limit on the reduction is
defined as 80% because a reduction in thickness of over 80% would result
in cracking during cold rolling. Thus, according to the present invention,
the reduction in thickness during cold rolling is preferably 20-80%.
(q) Annealing
In a preferred embodiment of the present invention after cold rolling
spheroidizing annealing may be carried out to soften the cold-rolled steel
sheet. The annealing conditions are defined as (Ac.sub.1 -50) to (Ac.sub.1
+30).degree. C. depending on the type of alloying elements added. When the
annealing temperature is lower than (Ac.sub.1 -50).degree. C., it takes a
very long time to achieve spheroidization of cementites, making the
process less efficient. On the other hand, when the temperature is higher
than (Ac.sub.1 +30).degree. C., the ferrite-pearlite structure once again
grows coarse, adversely increasing the length of heat treatment.
Furthermore, the strength of the material increases with a deterioration
in formability, i.e., pressing or forming into a final shape, which is
carried out by customers. One hour or longer of soaking is necessary for
spheroidizing precipitates. For this purpose box annealing is employed.
For the above reasons, in a preferred embodiment of the present invention,
the annealing is carried out by box annealing at a temperature of
(Ac.sub.1 -50) to (Ac.sub.1 +30).degree. C. for one hour or longer. It is
desirable to limit the processing period to at most 24 hours from the
viewpoint of increasing productivity.
Thin steel sheet produced by the process of the present invention is, after
purchased by customers, formed and shaped and then subjected to heat
treatment to provide a sufficient level of hardness and properties
required for the final products.
The present invention will be further described in conjunction with working
examples which are presented merely for illustrative purposes and do not
restrict the present invention in any way.
EXAMPLE 1
Steels A-H shown in Table 1 were hot rolled under the conditions indicated
as Process No. 1 in Table 2. From the resulting steel sheets, test pieces
each having a thickness of 1 mm and a V-shaped notch at the center of an
edge were cut. Austempering under the conditions shown in Table 3 was
performed on these test pieces to provide a tensile strength of 120
kgf/mm.sup.2. The thus-prepared test pieces were dipped into warm water at
50.degree. C. under a load of 60 kgf/mm.sup.2 to determine a length of
time until fracture occurred on the test piece.
The test results are shown in FIG. 1.
It can be seen that Steels A through E are superior to Steels F through H
with respect to resistance to cracking in the range of tensile strength of
150 kgf/mm.sup.2 or higher. Among the steels shown in Table 1, only Steels
A-E could satisfy target levels of 155 kgf/mm.sup.2, for tensile strength
and 55 hours for the length of time until cracking occurs.
EXAMPLE 2
In this example, Steels A, B and E of Table 1 were used. They had the same
levels of Mn and Cr but differed with respect to their carbon content. Hot
rolling was performed on these test steels under the conditions identified
as Nos. 1 through 8 in Table 2. Hot rolling conditions Nos. 1 through 4
fell within the range of the present invention.
A hydrogen embrittlement test was performed on the resulting hot rolled
steel sheet in the same manner as in Example 1. The results are shown
graphically in FIGS. 2 through 4. The reference numerals for each graph
indicate the number of the hot rolling conditions of Table 2.
FIG. 2 indicates tensile strength and the resistance to hydrogen cracking
for test pieces of steel A (0.34% of C) which were hot rolled under
conditions Nos. 1-8. When the steel was hot rolled under conditions Nos. 1
through 4, the hot rolled steel could satisfy the target levels of tensile
strength and the resistance to hydrogen cracking. However, the steel which
was hot rolled under conditions Nos. 5-8 did not satisfy those target
levels.
FIG. 3 indicates tensile strength and the resistance to hydrogen cracking
for test pieces of steel B (0.51% of C) which were hot rolled under
conditions Nos. 1-8. When the steel is hot rolled under conditions Nos. 1
through 4, some of the hot rolled steel satisfied the target levels of
tensile strength and 20 resistance to hydrogen cracking. However, the
steel which was hot rolled under conditions Nos. 5-8 did not satisfy the
target levels at all.
FIG. 4 indicates the tensile strength and the resistance to hydrogen
cracking for test pieces of steel B (0.68% of C) which were hot rolled
under conditions Nos. 1-8. When the steel was hot rolled under conditions
Nos. 1 through 4, some of the hot rolled steel satisfied the target levels
of tensile strength of 155 kgf/mm.sup.2 or higher and resistance to
hydrogen cracking of 55 hours or longer. However, the steel which was hot
rolled under conditions Nos. 5-8 did not satisfy the target level at all.
Thus, it is apparent from the foregoing that a thin steel sheet hot rolled
according to the present invention exhibits a remarkable improvement in
tensile strength as well as resistance to hydrogen cracking after
austempering.
EXAMPLE 3
In this example, Steels A through H of Table 1 were hot rolled under
conditions Nos. 1 through 4 of Table 2, and the resulting steel sheets
were subjected to cold rolling, and then box annealing under the
conditions specified in Table 4. The occurrence of cracking along the
edges during cold rolling and hardness after annealing were determined.
The results are shown in Tables 5 through 8.
It is apparent from the results that steels cold rolled and annealed under
conditions (a) through (d) of Table 4 were free from cracking at the edges
and the hardness after annealing reached a level of lower than HRB of 85.
In contrast, the steel which was cold rolled and annealed under conditions
(e) and (f) falling outside of the range of the present invention had a
hardness of higher than HRB of 85 because of a lower annealing temperature
or shorter annealing time. The steel processed under condition (g)
suffered from cracking at the edges for every steel composition and every
hot rolling condition because the reduction in thickness was too large.
Furthermore, the steel processed under annealing condition (h) exhibited a
hardness of larger than HBR of 85 because the reduction thickness was too
small and spheroidizing of cementite after annealing did not occur
sufficiently.
Thus, according to the preferred embodiments mentioned above, it is
possible to soften the steel without causing cracking at the edges by cold
rolling and annealing under conditions specified by the present invention.
EXAMPLE 4
In this example, Example 3 was repeated for the steels indicated in Table
9. After performing austempering on the resulting steels, the tensile
strength and resistance to hydrogen embrittlement were determined in the
same manner as in Example 1.
The results are shown in Table 10. A single asterisk shows the case in
which processing conditions fell outside the range of the present
invention and a double asterisk indicates the case in which processing
conditions fell outside the range of the preferred embodiments of the
present invention.
Although the present invention has ben described with preferred embodiments
it is to be understood that variations and modifications may be employed
without departing from the concept of the present invention as defined in
the following claims.
TABLE 1
__________________________________________________________________________
Steel
C Si Mn P S Cr Mo Ti Nb B sol. Al
N Ac.sub.1 Point
__________________________________________________________________________
(.degree.C.)
Present Invention
A 0.34
0.21
0.45
0.015
0.006
1.05
0.32
0.012
0.044
0.0012
0.044
0.0053
742.0
B 0.51
0.22
0.42
0.012
0.007
1.04
0.28
0.008
0.042
0.0013
0.048
0.0051
742.4
C 0.53
0.19
0.42
0.011
0.008
0.58
0.15
0.009
0.008
0.0007
0.048
0.0045
733.8
D 0.50
0.20
0.40
0.013
0.005
1.48
0.48
0.086
0.076
0.0018
0.047
0.0042
749.5
E 0.68
0.20
0.38
0.012
0.008
1.05
0.25
0.012
0.042
0.0014
0.046
0.0043
742.4
Comparative
F 0.49
0.21
*1.24
*0.032
0.018
1.12
0.45
0.011
0.035
0.0012
0.062
0.005
734.7
G 0.52
0.18
0.45
0.018
0.005
1.10
0.34
*0.002
0.014
0.0005
0.071
0.004
742.0
H 0.53
0.22
0.38
0.015
0.006
1.08
0.30
0.009
*0.003
0.0008
0.068
0.004
743.5
__________________________________________________________________________
NOTE:
*Outside the Range of the Present Invention.
TABLE 2
______________________________________
Finishing Cooling Rate
Coiling
No. Temp. (.degree.C.)
(.degree.C./sec)
Temp. (.degree.C.)
______________________________________
1 850 15 600
2 850 35 560
3 840 10 640
4 830 20 570
5 *780 20 620
6 860 *5 650
7 840 10 *690
8 850 *5 *500
______________________________________
NOTE:
*Outside the Range of the Present Invention.
TABLE 3
______________________________________
Soaking Conditions
845.degree. C., 8 min
Austempering Conditions
270.degree. C., 40 min; 310.degree. C., 40 min;
350.degree. C., 40 min; 390.degree. C., 40
______________________________________
min
TABLE 4
______________________________________
Reduction Annealing Holding Time
(%) Temp. (.degree.C.)
(h)
______________________________________
a 25 695 16
b 70 700 12
c 50 685 16
d 50 745 2.5
e 50 *630 12
f 50 700 *0.5
g *85 710 12
h 60 *820 *0.1
______________________________________
NOTE:
*Outside the Range of the Present Invention.
TABLE 5
__________________________________________________________________________
Hot Rolling Conditions No. 1
Cold Rolling,
Annealing Conditions
a b c d *e*f*g*h
__________________________________________________________________________
Present Invention Comparative
ABCDEFGH
.largecircle. .largecircle..largecircle. .largecircle..largeci
rcle. .largecircle..largecircle. .largecircle..largecircle.
.largecircle..largecircle. .largecircle..largecircle. .largecir
cle..largecircle. .largecircle.
.largecircle. .largecircle..largecircle. .largecircle..lar
gecircle. .largecircle..DELTA. .largecircle..DELTA.
.largecircle..largecircle. .largecircle..largecircle.
.largecircle..largecircle. .largecircle.
.largecircle. .largecircle..largecircle. .largecircle.
.largecircle. .largecircle..largecircle. .largecircle..
largecircle. .largecircle..largecircle. .largecircle..l
argecircle. .largecircle..largecircle. .largecircle.
.largecircle. .largecircle..largecircle. .largecir
cle..largecircle. .largecircle..largecircle.
.largecircle..largecircle. .largecircle..largecircl
e. .largecircle..largecircle. .largecircle..largeci
rcle. .largecircle.
##STR1##
__________________________________________________________________________
Left Side = Edge Cracking During Cold Rolling. (.largecircle.: No
Cracking, .DELTA.: Partially Occurred, X: Severe Cracking)
Right Side = Hardness After Cold Rolling and Annealing. (.largecircle.:
HRB < 85, X: HRB .gtoreq. 85)
*Outside the Range of the Present Invention.
A bold line area indicates the case in which satisfactory results were no
obtained.
This applies to Tables 6-8.
TABLE 6
__________________________________________________________________________
Hot Rolling Conditions No. 2
Cold Rolling,
Annealing Conditions
a b c d *e*f*g*h
__________________________________________________________________________
Present Invention Comparative
ABCDEFGH
.largecircle. .largecircle..largecircle. .largecircle..largeci
rcle. .largecircle..largecircle. .largecircle..largecircle.
.largecircle..largecircle. .largecircle..largecircle. .largecir
cle..largecircle. .largecircle.
.largecircle. .largecircle..largecircle. .largecircle..lar
gecircle. .largecircle..DELTA. .largecircle..DELTA.
.largecircle..largecircle. .largecircle..largecircle.
.largecircle..largecircle. .largecircle.
.largecircle. .largecircle..largecircle. .largecircle.
.largecircle. .largecircle..largecircle. .largecircle..
largecircle. .largecircle..largecircle. .largecircle..l
argecircle. .largecircle..largecircle. .largecircle.
.largecircle. .largecircle..largecircle. .largecir
cle..largecircle. .largecircle..largecircle.
.largecircle..largecircle. .largecircle..largecircl
e. .largecircle..largecircle. .largecircle..largeci
rcle. .largecircle.
##STR2##
__________________________________________________________________________
TABLE 7
__________________________________________________________________________
Hot Rolling Conditions No. 3
Cold Rolling,
Annealing Conditions
a b c d *e *f *g*h
__________________________________________________________________________
Present Invention Comparative
ABCDEFGH
.largecircle. .largecircle..largecircle. .largecircle..largeci
rcle. .largecircle..largecircle. .largecircle..largecircle.
.largecircle..largecircle. .largecircle..largecircle. .largecir
cle..largecircle. .largecircle.
.largecircle. .largecircle..largecircle. .largecircle..lar
gecircle. .largecircle..DELTA. .largecircle..DELTA.
.largecircle..largecircle. .largecircle..largecircle.
.largecircle..largecircle. .largecircle.
.largecircle. .largecircle..largecircle. .largecircle.
.largecircle. .largecircle..largecircle. .largecircle..
largecircle. .largecircle..largecircle. .largecircle. .
largecircle. .largecircle..largecircle. .largecircle.
.largecircle. .largecircle..largecircle. .largecir
cle..largecircle. .largecircle..largecircle.
.largecircle..largecircle. .largecircle..largecircl
e. .largecircle..largecircle. .largecircle..largeci
rcle. .largecircle.
##STR3##
.largecircle. .largecircle..largecircle.
.largecircle..largecircle. .largecircle..l
argecircle. .largecircle..largecircle.
.largecircle..largecircle. .largecircle..l
argecircle. .largecircle..largecircle.
.largecircle.
##STR4##
__________________________________________________________________________
TABLE 8
__________________________________________________________________________
Hot Rolling Conditions No. 4
Cold Rolling,
Annealing Conditions
a b c d *e*f*g*h
__________________________________________________________________________
Present Invention Comparative
ABCDEFGH
.largecircle. .largecircle..largecircle. .largecircle..largeci
rcle. .largecircle..largecircle. .largecircle..largecircle.
.largecircle..largecircle. .largecircle..largecircle. .largecir
cle..largecircle. .largecircle.
.largecircle. .largecircle..largecircle. .largecircle..lar
gecircle. .largecircle..DELTA. .largecircle..DELTA.
.largecircle..largecircle. .largecircle..largecircle.
.largecircle..largecircle. .largecircle.
.largecircle. .largecircle..largecircle. .largecircle.
.largecircle. .largecircle..largecircle. .largecircle..
largecircle. .largecircle..largecircle. .largecircle..l
argecircle. .largecircle..largecircle. .largecircle.
.largecircle. .largecircle..largecircle. .largecir
cle..largecircle. .largecircle..largecircle.
.largecircle..largecircle. .largecircle..largecircl
e. .largecircle..largecircle. .largecircle..largeci
rcle. .largecircle.
##STR5##
__________________________________________________________________________
TABLE 9
__________________________________________________________________________
Steel No.
C Si Mn P S Cr Mo Ti Nb B sol. Al
N Ac.sub.1 Point
__________________________________________________________________________
(.degree.C.)
1 0.54
0.18
0.36
0.012
0.006
1.08
0.37
0.012
0.043
-- 0.072
0.0043
742.7
2 0.32
0.21
0.35
0.011
0.008
1.04
0.38
0.012
0.043
0.0012
0.052
0.0046
742.9
3 0.68
0.21
0.38
0.012
0.007
1.03
0.37
0.013
0.038
0.0013
0.054
0.0048
742.4
4 0.48
0.12
0.37
0.011
0.006
1.04
0.37
0.013
0.037
0.0012
0.054
0.0043
737.1
5 0.56
0.64
0.37
0.012
0.007
0.97
0.39
0.013
0.041
0.0011
0.055
0.0044
754.0
6 0.56
0.21
0.13
0.006
0.007
0.98
0.37
0.015
0.039
0.0011
0.056
0.0045
744.2
7 0.54
0.20
0.95
0.011
0.007
0.98
0.38
0.014
0.038
0.0012
0.053
0.0047
735.2
8 0.55
0.21
0.38
0.018
0.018
0.98
0.37
0.014
0.038
0.0011
0.054
0.0046
741.6
9 0.56
0.19
0.37
0.013
0.008
0.52
0.13
0.015
0.039
0.0012
0.054
0.0046
733.3
10 0.54
0.18
0.38
0.014
0.008
1.86
0.48
0.015
0.042
0.0013
0.055
0.0045
755.6
11 0.63
0.21
0.36
0.013
0.007
1.02
0.32
0.006
0.041
0.0013
0.052
0.0045
742.4
12 0.48
0.21
0.39
0.013
0.009
1.02
0.34
0.094
0.042
0.0013
0.053
0.0047
742.1
13 0.52
0.19
0.39
0.013
0.006
1.02
0.41
0.021
0.008
0.0012
0.055
0.0044
741.5
14 0.54
0.19
0.38
0.012
0.005
1.01
0.40
0.016
0.078
0.0011
0.057
0.0043
741.5
15 0.54
0.19
0.39
0.011
0.007
1.03
0.40
0.015
0.038
0.0007
0.054
0.0041
741.7
16 0.55
0.19
0.38
0.011
0.007
0.99
0.40
0.015
0.038
0.0018
0.054
0.0048
741.1
17 0.53
0.20
0.38
0.012
0.006
0.98
0.38
0.016
0.039
0.0012
0.096
0.0046
741.3
18 0.55
0.21
0.38
0.012
0.008
0.98
0.39
0.016
0.041
0.0013
0.052
0.0038
741.6
19 0.55
0.20
0.39
0.013
0.009
1.02
0.38
0.015
0.042
0.0011
0.054
0.0135
741.8
20 0.48
0.20
0.41
0.012
0.009
1.01
0.37
0.015
0.043
0.0012
0.054
0.0061
741.5
21 0.53
0.18
0.40
0.012
0.008
0.97
0.40
0.016
0.041
0.0012
0.056
0.0063
740.3
22 0.53
0.19
0.41
0.011
0.007
0.98
0.39
0.014
0.039
0.0013
0.055
0.0063
740.7
23 0.54
0.21
0.41
0.011
0.006
0.99
0.38
0.014
0.039
0.0011
0.055
0.0064
741.4
24 0.53
0.21
0.32
0.012
0.008
0.99
0.37
0.014
0.038
0.0012
0.055
0.0062
742.4
25 0.55
0.20
0.38
0.012
0.007
1.04
0.39
0.016
0.035
0.0012
0.054
0.0065
742.3
26 0.56
0.21
0.38
0.013
0.009
1.04
0.40
0.017
0.040
0.0013
0.057
0.0062
742.6
27 0.51
0.18
0.39
0.014
0.007
1.02
0.40
0.015
0.041
0.0012
0.054
0.0060
741.3
28 0.52
0.18
0.40
0.012
0.006
1.00
0.35
0.015
0.040
0.0012
0.056
0.0062
740.8
29 0.55
0.19
0.38
0.013
0.009
1.00
0.38
0.014
0.042
0.0011
0.052
0.0061
741.3
30 0.54
0.20
0.39
0.012
0.008
0.97
0.38
0.016
0.040
0.0014
0.056
0.0063
741.0
31 *0.25
0.21
0.38
0.011
0.008
1.01
0.32
0.012
0.043
0.0008
0.052
0.0048
742.1
32 *0.83
0.20
0.37
0.011
0.008
1.03
0.34
0.011
0.041
0.0012
0.054
0.0043
742.2
33 0.45
0.21
0.38
0.012
0.007
0.99
0.41
*0.002
0.042
*0.0001
0.054
0.0044
741.7
34 0.45
0.19
0.36
0.012
0.009
0.95
0.40
*0.145
0.043
*0.0031
0.055
0.0095
741.2
35 0.60
0.18
0.39
0.013
0.006
0.98
0.40
0.016
*0.002
0.0013
0.056
0.0047
740.8
36 0.55
0.21
0.39
0.012
0.005
1.02
0.40
0.016
*0.132
0.0012
0.053
0.0046
742.1
37 0.55
0.21
0.38
0.012
0.007
1.01
0.38
0.015
0.038
0.0011
0.054
0.0046
742.1
38 0.48
0.19
0.39
0.011
0.007
0.97
0.39
0.015
0.037
0.0011
0.054
0.0045
740.7
39 0.53
0.19
0.38
0.011
0.006
0.98
0.38
0.016
0.041
0.0012
0.055
0.0045
741.0
40 0.49
0.19
0.38
0.012
0.008
0.99
0.37
0.014
0.039
0.0011
0.052
0.0047
741.1
41 0.51
0.19
0.38
0.012
0.009
0.99
0.40
0.014
0.038
0.0012
0.053
0.0044
741.1
42 0.50
0.20
0.39
0.013
0.009
1.04
0.39
0.014
0.038
0.0013
0.055
0.0043
742.2
43 0.54
0.21
0.41
0.014
0.008
1.04
0.38
0.016
0.039
0.0013
0.057
0.0041
742.3
44 0.55
0.20
0.40
0.012
0.007
1.02
0.37
0.017
0.042
0.0013
0.054
0.0045
741.7
45 0.54
0.20
0.41
0.013
0.006
1.00
0.39
0.015
0.041
0.0012
0.054
0.0046
741.3
__________________________________________________________________________
NOTE:
*Outside the Range of the Present Invention.
TABLE 10
Hot Rolling Conditions Cold Rolling-Annealing Conditions Resistance F
inishing Cooling Coiling Cold Rolling Annealing Holding Austempering
Conditions Tensile to Hydrogen Run Steel Temp. Rate Temp. Reduction
Temp. Time Soaking Autempering Strength Cracking No. No. (.degree.C.)
(.degree.C./sec) (.degree.C.) (%) (.degree.C.) (hr) (.degree.C. .times.
min) (.degree.C. .times.
min) (kgf/mm.sup.2) (hr) Remarks 1 1 850 25 590 50
675 20 870 .times. 10 330 .times. 40 186 125 Present 2 2 850 20 580
60 695 16 840 .times. 10 340 .times. 40 158 120 Invention 3 3 850 20
580 50 695 16 " " 193 98 4 4 840 15 600 50 700 16 " " 182 96 5 5 850
20 580 50 710 16 " " 189 82 6 6 840 20 600 45 695 16 " " 178 148 7
7 850 25 580 50 700 16 " " 186 57 8 8 845 20 600 50 700 16 " " 169 56
9 9 850 20 600 50 710 16 " " 164 137 10 10 850 25 580 60 710 16 " "
189 74 11 11 860 20 600 50 700 16 " " 186 72 12 12 845 15 620 55 700 16
" " 187 86 13 13 850 15 600 55 695 24 " " 184 58 14 14 855 15 620 55 695
24 " " 188 126 15 15 855 20 600 60 700 16 " " 185 117 16 16 855 20 620
50 700 16 " " 188 121 17 17 850 25 580 50 700 16 " " 181 84 18 18 860
25 580 60 695 24 " " 184 72 19 19 850 25 580 60 700 16 " " 192 84 20 20
820 15 600 45 695 24 840 .times. 10 340 .times. 40 186 56 21 21 850 35
600 50 700 16 " " 189 124 22 22 840 12 620 50 700 16 " " 183 60 23 23
830 20 640 50 695 24 " " 184 58 24 24 840 20 550 50 700 24 " " 187 146
25 25 860 25 610 70 690 24 " " 184 114 26 26 840 20 620 25 700 16 " "
185 120 27 27 830 25 600 40 695 16 " " 185 126 28 28 840 15 600 50 740
12 " " 187 112 29 29 845 20 600 50 710 2 " " 182 121 30 30 850 20 610
50 700 24 " " 186 124 31 31 840 20 600 50 700 18 840 .times. 10 340
.times. 40 128 206 Comparative 32 32 850 15 580 55 **695 16 " " 214
25 33 33 840 20 600 55 **695 16 " " 182 32 34 34 850 20 600 55 700 16 "
" 212 46 35 35 845 25 580 60 700 16 " " 184 16 36 36 850 20 600 50 710
16 " " 188 76 37 37 *780 20 620 50 700 16 " " 204 28 38 38 840 *60 600
60 700 16 " " 187 84 39 39 830 *5 620 60 700 24 " " 202 18 40 40 840 25
*720 45 **695 24 " " 203 16 41 41 860 20 *530 50 695 16 " " 186 96 42
42 840 15 620 **15 700 16 " " 184 84 Present 43 43 830 15 600 48 **620
12 " " 187 87 Invention 44 44 840 15 600 55 **800 **0.1 " " 182 92 45
45 845 20 600 50 700 **0.5 " " 186 91
NOTE:
*Outside the Range of the Present Invention.
**Outside the Range of the preferred embodiments of the Present Invention
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